The invention disclosed herein pertains to a machine working on a thermodynamic cycle which converts heat energy to work using heat added to the machine from a source external to the cylinders of the machine and serves as a heat pump when the machine is driven by an independent prime mover such as an electric motor.
A rudimentary Stirling engine is an example of such machine. It comprises two cylinders which each contain a piston A conduit connects the head ends of the cylinders together and there is a heat exchanger in the conduit The system is closed so the sum of the amount of gas in the two cylinders plus the conduit and heat exchanger is constant. One of the cylinders is maintained at a low temperature, that is, it rejects heat during a compression phase of the cycle and the other cylinder is maintained at a high temperature and it absorbs heat during an expansion or decompression part of the cycle. Piston movements are such that gas is transferred back and forth between the hot and cold cylinders. When more of the gas is in the hot cylinder, the pressure rises and when the gas is transferred back to the cold cylinder the pressure falls again. When operating in the engine mode, the ideally isothermal compression stroke starts with most of the working gas in the cold cylinder of the engine. After the compression stroke is near completion the piston in the hot cylinder moves in a direction to expand the volume in the hot cylinder which results in transfer of the gas to the hot cylinder. Because external heat is applied to the hot cylinder the pressure in that cylinder rises, the gas is expanded and the piston in the hot cylinder does useful work. At any given position of the piston in the cold cylinder, the pressure is higher on the outward stroke with some of the gas hot than it is on the inward stroke when all of the gas is cold. Hence, more work is done on the piston in the cold cylinder during gas expansion than has to be done to recompress the gas and the difference is the net work available from the engine. When gas is transferred from the cold cylinder to the hot cylinder the gas absorbs heat from the heat exchanger which is usually called a regenerator. When gas is transferred from the hot cylinder to the cold cylinder the regenerator removes heat from the gas so the gas enters the cold cylinder at a temperature substantially lower than that which existed in the hot cylinder.
In a practical engine, there may be several pairs of hot and cold cylinders adjacent each other with their pistons connected to a common crankshaft and their cylinders are connected in series. According to conventional practice, heat of combustion such as from a gas flame is applied to the hot cylinder at all times. To obtain the best results the hot gases of combustion are made to flow around the hot cylinder to provide the best opportunity for heat transfer to the gas inside.
The same situation exists when the engine is operated in the heat pump mode wherein the crankshaft is driven rotationally by an independent prime mover such as an electric motor. In the heat pump mode, heat conducted outwardly from the so called cold cylinder is sometimes absorbed in water contained in a jacket which surrounds the cold cylinder. Alternatively, the heat may be absorbed in an air stream passing around the cold cylinder. The hot water or hot air may be used to heat a building or to perform an industrial process. Compression of the gas in the cold cylinder would be an isothermal compression ideally if all of the heat generated in the gas due to compression were transferred to the water or air surrounding the cold cylinder.
It will be evident that it would be highly advantageous to maximize the amount of heat per unit of time absorbed by the gas in the hot cylinder when the machine is being operated as an engine and to maximize the amount of heat that is extracted from the gas in the cold cylinder when the machine is being operated as a heat pump to improve the efficiency or coefficient of performance of the machine and obtain an operating cost benefit in either mode.